Elastic laminate

ABSTRACT

An elastic laminate has a coextruded film having an outer film layer formed of an elastic and sticky styrene-block copolymer and an inner film layer of lesser elasticity than the outer layer, an outer fleece or nonwoven web or overlay, an outer adhesive layer between the outer fleece web and the outer film layer adhering the outer fleece web to the outer film layer, an inner fleece or nonwoven web or overlay, and an inner adhesive layer between the inner fleece web and the inner film layer adhering the inner fleece web to the inner film layer. The inner adhesive layer forming between the inner fleece web and the inner film layer a bond that is weaker than a bond formed by the outer adhesive layer between the outer fleece web and the outer film layer such that on stretching the inner fleece layer separates locally from the inner film layer.

FIELD OF THE INVENTION

The present invention relates to an elastic laminate.

BACKGROUND OF THE INVENTION

An elastic laminate is known having a film on both its faces bymonolayer or multilayer nonwoven overlays. The elastic laminate isprovided in particular for parts of hygiene articles subject to highstresses, such as elastic closing strips on disposable diapers, forexample.

Styrene block copolymers are characterized by good elastic propertiesand are therefore suitable for producing elastic laminates that aresubject to high stresses.

Because of the very high elasticity, styrene block copolymers are verysticky and can therefore be processed as monofilaments only with greateffort. In addition to an elongation that is difficult to control duringprocessing of such a film, it cannot easily be rolled up into a rollbecause then there is an increased risk of sticking together.

DE 298 25 018 (U.S. Pat. Nos. 6,159,584 and 6,531,207) describes astretchable elastic strip having a coextruded elastic film. The elasticfilm comprises an elastic layer as well as at least one additionalinelastic layer. In the case of the stretchable elastic strip, thecoextruded film is covered by a nonwoven overlay on at least one face,producing stretchable and unstretchable regions in specific zones. It ispossible to provide here for the nonwoven overlay to be connected to thecoextruded film in fastening regions that are spaced apart from oneanother in the stretchable regions, thereby forming raised archedregions between the fastening regions. Due to the alternation betweendefined fastening regions and arched regions, this forms a definitelyperceptible and recognizable wave shape that impairs the feel and thetextile character of the material.

OBJECTS OF THE INVENTION

It is therefore an object of the present invention to is provide animproved elastic laminate.

Another object is the provision of such an improved elastic laminatethat overcomes the above-given disadvantages, in particular that has auniform surface that is especially soft on one face.

SUMMARY OF THE INVENTION

An elastic laminate has according to the invention a coextruded filmhaving an outer film layer formed of an elastic and sticky styrene-blockcopolymer and an inner film layer of lesser elasticity than the outerlayer, an outer fleece or nonwoven web or overlay, an outer adhesivelayer between the outer fleece web and the outer film layer adhering theouter fleece web to the outer film layer, an inner fleece or nonwovenweb or overlay, and an inner adhesive layer between the inner fleece weband the inner film layer adhering the inner fleece web to the inner filmlayer. The inner adhesive layer forming between the inner fleece web andthe inner film layer a bond that is weaker than a bond formed by theouter adhesive layer between the outer fleece web and the outer filmlayer such that on stretching the inner fleece layer separates locallyfrom the inner film layer.

The inner and outer adhesive layers are applied as continuous layerscovering the entire confronting faces of the respective film layers andfleece webs. But since the inner adhesive has a weaker bond, if thelaminate is stretched there will be local separation between the innerfilm layer and the inner fleece web and inelastic stretching of theinner web so that when the stretching tension is released, there will bebunching of the inner fleece web to form a very soft and continuousinner surface on the laminate. These local separations can be betweenthe inner fleece layer and the inner adhesive layer, or between theinner film layer and the inner adhesive layer.

Before such activation by stretching, both the outer fleece web on theouter film layer and the inner nonwoven overlay web having the innerfilm layer are joined over the full region by the respective adhesivelayer. The adhesion is adjusted so that in activation by an initialstretching, the inner film layer is separated from the inner adhesivelayer and/or the outer film layer in some regions. The adhesion betweenthe inner film layer and the inner adhesive layer is usually also lowerthan the cohesive bonding forces within the inner adhesive layer and isalso lower than the adhesion between the adhesive layers on the one handand the respective nonwoven overlay on the other hand. Then in theelongation an adhesion failure occurs between the inner film layer andthe outer film layer and/or the inner film layer and the inner adhesivelayer. Due to the separation occurring in some layers within the layercomposite, a microstructure without a fixed pattern is created,resulting in minor warping distributed over the entire region of theelastic laminate under the inner nonwoven overlay, thereby improving thehaptic properties of the inner nonwoven overlay. Thus, in comparisonwith a design in which there is no separation of layers in some regions,a softer, more voluminous surface is created with the same weight perunit of region of the inner nonwoven overlay.

The nonwoven overlay web may also be formed by one layer or multiplelayers of a carded nonwoven, a spunbond nonwoven or a melt-blownnonwoven, and multilayer combinations of different nonwoven materialsmay also be considered in particular. Polyolefins such as polyethyleneor polypropylene, polyolefin copolymers or polyolefin blends arepreferred as the basic substance of the fibers because of the low costfor disposable items in particular.

The present invention is based on the discovery that, by adjusting theadhesion at the interfaces of the inner film layer, the feel or hand canbe improved for the inner nonwoven overlay. The layer separationaccording to the invention may also be accomplished by suitablycoordinating the polarities of the individual layers, but in general,great differences in polarity lead to a reduction in adhesion. Thestyrene block copolymer as a material for the outer film layer has amoderate polarity. In addition, according to a preferred embodiment ofthe invention, a hot-melt adhesive based on styrene block copolymer, inparticular styrene-isoprene-styrene block copolymer (SIS), is used asthe material for the adhesive layers, such that basically both adhesivelayers may be formed from the same adhesive. In addition, adhesivesbased on polyurethane (PU) may also be used. To achieve local adhesionfailure in some regions on one of the interfaces of the inner filmlayer, a material that definitely differs in polarity from the adjacentlayers is advantageously selected. Since the styrene block copolymershave a moderate polarity, either strongly polar polymers and polymershaving a low polarity or apolar polymers are suitable for the inner filmlayer.

Suitable polymers having a strong polarity include, for example,polyamides (PA) with their strongly polar amide group, polymethylmethacrylate (PMMA), polyoxymethylene (POM) and ester thermoplastics,such as polycarbonate (PC), polyethylene terephthalate (PET) andpolybutylene terephthalate (PBT). In addition, at least two of theabove-described polymers and/or different types of the above-describedpolymers may also be provided as a mixture.

Depending on the layer composition, the desired separation may also beachieved with a moderately polar polymer in combination with an adjacentlayer of a polymer that is apolar or has a low polarity. For example,polystyrene (PS), whose moderate polarity can be attributed to thep-electron system of the aromatic constituents, still has a high enoughpolarity, depending on the material combination.

Materials having a low polarity or none at all include in particularpolyolefins, such as polyethylene (PE) and polypropylene (PP).

The polymers are selected so that the polarity difference between theouter film layer and the outer adhesive layer is lower than the polaritydifference between the outer film layer and the inner film layer, on theone hand, and the polarity difference between the inner film layer andthe inner adhesive layer, on the other hand.

The internal structure of the laminate after activation depends inparticular on the material properties of the inner film layer and thetype of activation process. An elongation of at least 50% usually occursin activation, but an elongation of at least 100% is preferred, and amuch greater increase in length may also be provided. The activation mayoccur in only one direction to establish a preferred direction ofelongation. In particular there is also the possibility of creating amaterial that is essentially stiff with moderate tensile forcesperpendicular to the direction of activation.

According to a first additional embodiment, the film layer remainscompletely continuous and imperforate even after activation. Within thescope of such an embodiment, the inner film layer undergoes plasticdeformation during the activation without tearing. A section-by-sectionseparation of layers may take place with respect to the outer film layeras well as the inner film layer.

According to an alternative embodiment, the inner film layer has breaksthat are formed when the inner film layer cannot withstand the tensileforces during activation and therefore tears. Since the inner film layeris supported and guided to a certain extent by the inner nonwovenoverlay adjacent over the inner adhesive layer as well as by the outerfilm layer, microtears, which are usually distributed over the surface,occur without completely tearing the inner film layer.

The coextruded film that preferably has only two layers usually has atotal thickness between 12 μm and 150 μm. The thickness of the outerelastic film layer is usually between 10 μm and 100 μm, preferablybetween 25 μm and 70 μm. The inner film layer usually has a thicknessbetween 2 μm and 50 μm, preferably between 5 μm and 20 μm. In thiscontext it should be taken into account that the elastic properties ofthe laminate are determined essentially by the outer elastic film layerand restrictions in elasticity due to the inner film layer should beavoided. In the case of an adequate strength of the inner film layer, itmay also stabilize the coextruded film as an intermediate product duringprocessing, such that a great elongation of the coextruded film can beavoided during handling of the coextruded film under tension. Inparticular a coextruded film may also be rolled up and stored as anintermediate product because the inner nontacky film layer as aseparation between the layers of the outer film layer on top of oneanother prevents blocking.

The adhesion may be determined according to DIN 53357: 1982.

Strips may be cut from the elastic laminate and may be used to aparticular extent as elastic closing strips on diapers.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features, and advantages will become morereadily apparent from the following description, reference being made tothe accompanying drawing in which:

FIG. 1 is a large-scale section through the laminate according to theinvention; and

FIG. 2 is a view like FIG. 1 of another laminate in accordance with theinvention.

SPECIFIC DESCRIPTION

As seen in FIG. 1, an elastic laminate has a two-layer coextruded film 1having an outer tacky film layer 2 of styrene block copolymer and aninner film layer 3 having a lower elasticity than the outer film layer2. An outer fleece web 4 is bonded to the outer film layer 2 over thefull region by an outer adhesive layer 5.

An inner adhesive layer 7 is arranged between the inner film layer 3 andan inner nonwoven overlay 6 and is detached from the inner film layer 3in some regions but still adheres to it. This local separation isachieved by an initial elongation of the laminate with a 100% increasein length, for example. In the case of essentially complete restorationof the laminate, based on the elastic properties of the outer film layer2, the material contracts again, such that the inner film layer 3 isseparated from the inner adhesive layer 7 in some regions due to anadhesion failure because of the low adhesion at the joint interface. Dueto the deformations of the inner nonwoven overlay 6 and the inneradhesive layer 7, the inner nonwoven overlay 6 becomes more voluminouson the whole without revealing a discernible macroscopic structure to auser. Instead, a pleasant feel is achieved even with a low weight perunit of region of the inner nonwoven overlay 6.

FIG. 2 shows an embodiment having a basically comparable layerstructure, where the material of the inner film layer is selected andthe activation is performed in such a way that the inner film layer 3tears in some regions, so that breaks 8 remain in the inner film layer 3after the activation. In addition, the polarity of the inner film layeris coordinated with the material of the outer film layer in such a waythat there is separation in some regions at the interface between thefilm layers 2 and 3. As described previously with respect to FIG. 1, thelocal separation produces a microstructuring below the inner nonwovenoverlay 6, so that the latter gives a softer and more voluminous effect.

1. An elastic laminate comprising: a coextruded film having an elasticouter film layer formed of a styrene-block copolymer and an inner filmlayer of lesser elasticity than the outer layer; an outer fleece web; anouter adhesive layer between the outer fleece web and the outer filmlayer adhering the outer fleece web to the outer film layer; an innerfleece web; and an inner adhesive layer between the inner fleece web andthe inner film layer adhering the inner fleece web to the inner filmlayer, the inner adhesive layer forming between the inner fleece web andthe inner film layer a bond that is weaker than a bond formed by theouter adhesive layer between the outer fleece web and the outer filmlayer such that on stretching the inner fleece layer separates locallyfrom the inner film layer.
 2. The elastic laminate defined in claim 1wherein the inner and outer adhesive layers are of the same polymer. 3.The elastic laminate defined in claim 2 wherein the inner and outeradhesive layers are made of a hot-melt adhesive based on a styrene blockcopolymer.
 4. The elastic laminate defined in claim 1 wherein the innerfilm layer is made of a polyamide, polymethylmethacrylate,polyoxymethylene, polycarbonate, polyethylene terephthalate, orpolybutylene terephthalate or mixtures thereof.
 5. The elastic laminatedefined in claim 1 wherein the inner film layer is made of polyethyleneor polypropylene.
 6. The elastic laminate defined in claim 1 wherein apolarity difference between outer film layer and the outer adhesivelayer is less than the polarity difference between the inner film layerand the outer film layer and the polarity difference between the innerfilm layer and the inner adhesive layer.
 7. The elastic laminate definedin claim 1 wherein the fleece webs are of substantially identicalcomposition and properties.
 8. The elastic laminate defined in claim 7wherein the outer film layer has a thickness between 10 μm and 100 μm.9. The elastic laminate defined in claim 8 wherein the inner film layeris has a thickness between 2 μm and 50 μm.
 10. The elastic laminatedefined in claim wherein the coextruded film has only two layers. 11.The elastic laminate defined in claim 1 wherein the working temperaturesof the polymers of the outer film layer, the inner film layer, and boththe adhesive layers is between 180° C. and 260° C.
 12. The elasticlaminate defined in claim 1 wherein the inner film layer is imperforateand continuous.
 13. The elastic laminate defined in claim wherein theouter film layer is formed with breaks forming throughgoingperforations.